The present invention relates generally to high speed data transmission systems that use multi-carrier modulation. More particularly, an express parameter changing command and protocol suitable for use in multi-carrier transmission systems is disclosed.
In recent years, there has been increased interest in the use of multi-carrier modulation in high speed modems. By way of example, the Alliance For Telecommunications Information Solutions (ATIS), which is a group accredited by the ANSI (American National Standard Institute) Standard Group, has promulgated a discrete multi-tone based standard for the transmission of digital data over Digital Subscriber Lines (ADSL). The standard is intended primarily for transmitting data over ordinary telephone lines, although it may be used in a variety of other applications as well. The North American Standard is referred to as the ANSI T1.413 ADSL Standard and is incorporated herein by reference. Transmission rates under the ADSL standard are intended to facilitate the transmission of information at rates of up to at least 6 million bits per second (i.e., 6+ Mbit/s) over twisted-pair phone lines. The standardized system defines the use of a discrete multi tone (DMT) system that uses 256 xe2x80x9ctonesxe2x80x9d or xe2x80x9csub-channelsxe2x80x9d that are each 4.3125 kHz wide in the forward (downstream) direction. In the context of a phone system, the downstream direction is defined as transmissions from a central office (typically owned by the telephone company) to a remote location that may be an end-user (i.e., a residence or business user).
Although the ADSL standard has been widely accepted, there are ongoing efforts to both improve the T1.413 ADSL standard and to provide ADSL or other standards for communications at other data rates. By way of example, there are currently ongoing efforts to define a simplified version of the standard which, among other things uses just 128 tones. This effort is being undertaken by T1.413 and is commonly referred to as the G.lite standardization effort. There is also an effort to define a standard for significantly higher data, which is referred to as the VDSL (Very High Rate Digital Subscriber Line) standard. The VDSL standard is intended to facilitate transmission rates of at least 25.96 Mbit/s and preferably at least 51.92 Mbit/s in the downstream direction. To achieve these rates, the transmission distance over twisted pair phone lines must generally be shorter than the lengths permitted using ADSL. Simultaneously, the Digital, Audio and Video Council (DAVIC) is working on a similar system, which is referred to as Fiber To The Curb (FTTC). The transmission medium from the xe2x80x9ccurbxe2x80x9d to the customer premise is standard unshielded twisted pair (UTP) telephone lines.
One issue that is inherent in high speed DSL modems that use multi-carrier modulation is how to handle variations in line conditions. For example, in the T1.413 standard and other proposed DMT based systems, the communicating modems go through a brief training period before data communications begin. During the training period test signals are transmitted to effectively test the quality of the line at various frequencies. Generally the line quality is determined by measuring the signal-to-noise ratio (SNR) on each of the tones. The number of xe2x80x9cbitsxe2x80x9d allocated to each tone are then determined based in large part on the detected training signals. However, after the training period, the transmission line often encounters changes that may affect its ability to transmit information at the allocated rates on some of the tones. The transmission line changes may result from a variety of causes including: a customer taking a phone off its hook or hanging up; temperature induced line changes; changes in cross talk noise due to adjacent lines becoming active or inactive, and the increase of AM radio signals at night.
If the line quality deteriorates over time, then more errors are likely to occur and something must be done to adjust the assigned bit allocation. One way to adjust the bit allocation is to simply retrain the modems. However, retraining has the drawback of taking a relatively large amount of time which causes a brief service interruption. The other way to adjust the bit allocation defined in the T1.413 standard is a procedure referred to as xe2x80x9cbit swappingxe2x80x9d. The bit swapping protocol contemplates that when errors are detected on a particular tone, the amount of information transmitted on that particular tone will be reduced by some number of bits. If another tone is believed to have additional SNR, the amount of information transmitted is increased by a corresponding amount.
In the T1.413 standard, the bit swapping protocol is specifically defined. More specifically, when a particular receiver determines that a bit swap needs to be made, it send a bit swap request over an overhead channel (typically referred to as the AOC-ADSL Overhead Channel). The bit swap request has a designated format which is illustrated in FIG. 1. As seen therein, the first byte of the bit swap request is a message header 12. Message header 12 consists of all ones, which identifies the command as a bit-swap request. The message header 12 is followed by a message 14 of eight (or twelve) bytes. The message 14 is divided into 4 (or six) segments, each segment is called a message field 16. Each message field 16 contains a one byte command 18 followed by a one byte tone index 20 which identifies the tone to which the command is to be applied. The one byte command includes the functions: add a bit, delete a bit, increase power by 1, 2, 3 dB, decrease power by 1, 2, 3 dB, do nothing, and proprietary commands
The T1.413 further requires that a 3-byte bit-swap acknowledge command be sent back to the unit requesting the bit swap to confirm receipt of the bit swap request. The bit-swap acknowledge command specifies a specific symbol count on which the swap will be implemented. The acknowledgement command is used to simplify detection of the implementation of a new bit distribution, but the acknowledgement, slows swapping speed and still may cause a failure if the acknowledgement is not received.
The T1.413 protocol further mandates that the bit-swap request command be transmitted five successive times and that the receiving unit only acknowledge the bit swap command if it receives a majority of those five transmissions. Thus, it takes 45 bytes to request to move one bit plus 15 bytes to acknowledge the request (which also must be repeated 5 times). Ignoring latencies, the minimum time for a swap is thus on the order of 30 ms (60 bytes at 16 kbytes/sec). However, current standards actually mandate that swapping occurs no more often than once every 800 ms, which allows for transceiver simplification, but further slows the swapping process in situations where more then four tones are to be changed. Thus, the standard ADSL bit-swapping mechanisms permit slow variations of the transmitter. However, with the advent of splitterless ADSL and the increasing popularity of DSL in general, it has become apparent that DSL lines will be subject to abrupt changes that require significant changes in the bit distribution. Given the slowness of the standardized bit swapping protocol and the drawbacks of requiring a retraining event, it has become apparent that a more efficient mechanism is needed to reduce the time it takes to implement bit redistribution in multi-carrier transmission systems.
To achieve the foregoing and other objects of the invention methods and devices for adaptively changing a parameter (such as sub-carrier gain or bit allocation) of a communication signal in a multi-carrier based transmission system are described. In one aspect of the invention, a unit that determines a need for a change sends a change request to a second unit. The change request identifies one or more specific sub-carriers to be altered and a desired value for a parameter associated with each identified sub-carrier. The requesting unit then monitors the communication signal it receives to determine whether the requested change has been implemented. The determination of whether the requested change has been implemented is based at least in part upon an analysis of a portion of the received communication signal that was intended to be changed.
In preferred embodiments, the change request is suitable for identifying a plurality of specific sub-carriers to be altered, as well as, a desired value for a parameter associated with each identified sub-carrier. By way of example, the parameter might be a desired bit allocation or a desired power level (gain) for the associated sub-carrier. The desired value may take the form of an absolute value (e.g. transmit 8 bits on this tone) or a relative value (e.g. increase the number of bits transmitted on this tone by 2). In another preferred embodiment, the protocol does not include any explicit acknowledgement that the change request was received or implemented.
The monitoring may be done in a wide variety of ways. By way of example, in some embodiments, the requesting unit redundantly decodes the received communication signal using both the current value of the parameter and the desired value of the parameter. In this embodiment, the determination of whether the requested change was implemented is based at least in part upon the decodings. By way of example, the determination of whether the requested change has been implemented may be based at least in part upon analyzing errors detected using each decoding and selecting the decoding that generates less errors. Another approach is to generate a first forward error correction syndrome based upon the current value decode and a second forward error correction syndrome based on the desired value decode. In this approach the determination of whether the change request was implemented is based on an analysis of the syndromes.
In some embodiments, the monitoring includes monitoring one or more of the specific sub-carrier(s) that are intended to be changed by the change request. If a change is detected on the specific sub-carriers, it is assumed that the change has been implemented. Again, this can be done using a wide variety of mechanisms. By way of example, when the multi-carrier signal is a DMT signal, the energy level of one or more of the tones can be monitored. For example, the change might increase the allowable power for a particular tone. In this scenario, if more energy is detected on that particular tone than would be expected using the current parameter values, then it can be assumed that the requested change had been implemented. Alternatively, one of the tones could be zeroed or a zeroed tone could be activated. These types of approaches are both relatively easy to implement and make it easy to detect the implementation of a change without requiring explicit feedback from the unit requesting the change.
In another aspect of the invention, the change request command includes a header, an express swap control, at least one sub-carrier identifier, at least one desired parameter value indicator, and an error field. The header identifies the command as a change request command. The express swap control specifies the changed tone count, which indicates the number of tones to be altered by the change request command. Each sub-carrier identifier identifies a specific sub-carrier to be altered by the change request command. Each desired parameter value indicator identifies a desired value of a parameter of its associated sub-carrier. The error field permits the unit receiving the change request to detect whether there is an error in its interpretation of the change request command. In some embodiments, a super frame number may be included in the header. In other embodiments, a super frame number may be included in the express swap control. Further, the super frame number specifies how many super frames later or on which super frame the express swap should occur.
In a preferred embodiment, the desired parameter value indicator identifies at least one of a desired bit allocation and a desired gain for its associated sub-carrier. In some embodiments, a single byte can be used to identify both the desired bit allocation and the desired gain for a particular sub-carrier.
In another aspect of the invention, an improved modem design that includes redundant decoders within the receiver is described. The redundant decoder are configured to decode a demodulated multi-carrier signal using different parameters for at least one of the subcarriers that is to be changed by a change request command. An analyzer is then provided to determine which of the redundant decoders has decoded the correct signal.
In one embodiment, the redundant decoders are subcarrier decoders arranged to decode the same subcarrier. In another embodiment, the redundant decoders are signal decoders arranged to decode a plurality of the subcarriers of the multi-carrier signal. In one specific implementation the analyzer includes redundant syndrome generators that are arranged to generate the syndromes of the signals decoded by the redundant decoders.